The present invention relates to the art of gas permeation reduction. It finds particular application in conjunction with reducing hydrogen loss in Stirling engines by establishing and maintaining a hydrogen permeation barrier on the inner surface of the engine heat tubes and will be described with particular reference thereto. However, it is to be appreciated that the present invention will have other applications particularly where hydrogen or other highly permeable gases function as the working fluid.
Stirling engines achieve the greatest efficiency with a low molecular weight gas for the working medium, e.g. hydrogen or helium. The relative rarity and high cost of helium gas render its use impractical for commercial engines in which a large volume of working medium or gas would be required. Hydrogen, which is both more abundant and lower in molecular weight than helium, readily permeates the metal alloys and other materials of which Stirling engines are commonly constructed. The loss of hydrogen by permeation through these metal alloys at the relative high temperature and pressures of a Stirling engine are generally considered to be unacceptably high.
A relatively small partial pressure of carbon dioxide, carbon monoxide, or water vapor as a dopant has been found to be effective in reducing the loss of hydrogen by permeation. However, the temperature and pressure variations within a Stirling engine are sufficiently great that the water vapor tends to condense in the cooler regions of the engine. The condensed water vapor causes severe mechanical and corrosion problems. During the engine cycle, the carbon dioxide and monoxide dissociate, freeing oxygen which combines with the hydrogen to form additional water vapor. The increasing concentration of water vapor, again, condenses in regions of lower temperature or higher pressure.
Commonly, carbon monoxide and carbon dioxide are effective in reducing hydrogen permeation over a partial pressure range of 0.1 to 1.0%. To avoid condensation problems, the water vapor must be present in partial pressures that are sufficiently low that condensation is avoided. The exact partial pressure, of course, varies with the operating temperature and pressure of the engine. However, partial pressures which are lower than 0.1% are normally required.
Other approaches for reducing hydrogen loss are illustrated in U.S. Pat. No. 4,197,707 issued Apr. 15, 1980 to Kenji Asano, which has a recovery system for recapturing lost hydrogen which has escaped from the system. U.S. Pat. No. 4,335,884 issued June 22, 1982 to Michael Darche limits hydrogen loss between the pistons and cylinder walls through the use of flexible or rolling diaphragms. Both of these techniques, however, are limited in their ability to stop hydrogen loss.
In accordance with the present invention, a method and apparatus is provided for accurately maintaining dopant levels in the working fluid of Stirling engines and the like.